vxlan: support for ndo_fdb_get

This patch implements ndo_fdb_get for a vxlan device.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Reviewed-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>